Standard type optical fibers comprise a cladding having an essentially circular cylindrical outer cladding surface and a thin fiber core, which is located rather centrally inside the cladding and in the ideal case is located along the longitudinal axis of the outer cylindrical surface and thus has the same longitudinal axis as this.
Optical fibers having twin cores, where each core has a shape similar to the core of standard fibers, and the two cores are located essentially symmetrically along a diameter plane of the circular cylindrical cladding, constitute a material used in the research and utilization of many linear and non-linear phenomena, which are based on interaction of the evanescent fields of the fundamental propagational modes of the cores. Such phenomena are used in beam splitters, fiber sensors and non-linear switches.
A major drawback associated with the use of such fibers is however the difficulty both in the excitation and the detection of the signals in the two cores due to their small size and due to the fact that they are located comparatively close to each other. A typical core radius in a fiber having twin cores is approximately 3-4 .mu.m and a typical distance between the two cores is of the magnitude of order of a few radii of the core. It is impossible to obtain a butt joint between an optical fiber having a single core of standard type and a fiber having twin cores and between two fibers having twin cores by means of the conventional splicing methods.
A method, which has been used to overcome this problem, is to use large and powerful optical elements and lenses to focus the incoming light to the cores. Such methods, however, suffer from high losses (7-8 dB) at the introduction of the light, which together with the inconvenience of using larger optical components, for instance due to their unsatisfactory stability during practical use, make them inconvenient for practical use.
In Swedish patent SE-C2 500 874, "Alignment of and splicing optical PM-fibers", which is incorporated by reference herein, it is disclosed how an optical PM fiber can be given a predetermined angular alignment along its longitudinal axis and how this alignment can be used to provide good joints between two optical PM fibers. In the determination the fiber is illuminated with light and the lens effect is then observed, i.e. the light intensity is determined for light passing through the fiber. A light intensity curve perpendicular to the axis of the fiber has then generally a maximum, which corresponds to the core or to the central region of the optical fiber. Outside this maximum there is a region having a lower light intensity, but where the light intensity still can be fairly constant, on said line. Regions outside the outer surface of the fiber obtain a light intensity approximately corresponding to the light intensity without any fiber at all placed in the light beam path. The lens effect is constituted by the contrast between the central region having a high light intensity and the region which is located closest thereto. For an alignment the fiber is rotated, so that the lens effect either becomes maximal or minimal.
In the published International patent application WO-A1 95/14945, "Determination of angular offset between optical fibers having optical, axial asymmetry and alignment and splicing of such fibers", which is also incorporated by reference herein, image analysis is used having a refined evaluation procedure of light intensity curves captured perpendicularly to the longitudinal directions of the fibers in order to for instance splice fibers having twin cores to each other.